The objectives of this research are to determine where genes act in the mammalia central nervous system, how cells interact and when and where cells intermingle in the developing CNS. Experimental mouse chimeras containing mixtures of genetically normal and genetically mutant cells determine the genotype of individual cells in the chimeras regardless of their phenotypic appearance. In this way it will be determined in which cell type(s) various mutant genes are acting. The mutants and mutant reversibly yields normal chimeras will be examined histologically, histochemically and with the electron microscope to determine how normal and mutant cells interact. The studies will also include cell interactions between normal cells that differ in enzyme activity to examine the phenomenon of enzyme transfer. Neuronal migration and mosaic patterns in the central nervous system will be examined. If the data continue to indicate extensive cell mixing, various developmental stages will be studied to determine when the cell mixing occurs.